Galaxy Mergers: Now with Feedback from Massive Stars

These movies shows the gas and stars in a pair of gas-rich merging galaxies (similar to the galaxy M82), as they collide on a relatively “gentle” orbit in their major merger. Giant molecular clouds (GMCs) continuously form as gas cools, and collapses to high densities in the wake of strong shocks driven by the rapidly varying gravitational potential of the colliding systems. In the final coalescence, the star formation rates reach ~100 solar masses per year. In the wake of each burst of new stars formed, feedback from young massive stars -- a combination of radiation pressure pushing on dust and supernovae explosions heating up gas -- blows powerful winds that escape the galaxies.

Left: This movie shows the starlight in the merger (as in the sample frames above). Each frame is a mock three-color optical image (Sloan ugr bands). Young star clusters which have blown away their natal molecular clouds appear as blue points; obscuration by gas and dust appears as the dark red/brown “dust lanes.”

These movies show the evolution of a merger in which the initial galaxies are built to closely resemble the Milky Way. The same processes occur, but on a (fractionally) smaller scale, because the galaxies are gas-poor.

Milky Way Merger: Gas: This movie shows the gas in the merger, with the same color scale as in the (gas) movie for the gas-rich merger above.

Milky Way Merger: Stars: This movie shows the starlight in the merger, with the same mock optical image scale as used in the movie of the gas-rich merger above.

The looping gif here shows iso-density contours of the gas during the merger. As it spins you can get a 3D sense of the structure of the tidal features created by galaxy mergers. Credit: Desika Narayanan

The initial galaxies here are low-mass dwarf galaxies (similar to the Small Magellanic Cloud). In these small galaxies, the gravitational potential is relatively weak, so the super-winds expelled by feedback can contain ~10-20 times the mass turning into stars. This leads to the explosive outflows during the final stages in the merger, when the star formation rate increases by a large factor and removes a significant fraction of the galaxy mass.

Here, the initial galaxy model represents an extremely massive, high redshift starburst disk. Stars form at ~1000 solar masses per year during the peak of the merger-induced starburst, comparable to the most luminous systems in the Universe. In the extreme gas densities found here, thermal energy deposited by supernovae is radiated away almost immediately, but the massive flux in starlight scattering off of dust and gas drives powerful winds. As with the isolated (non-merging) simulations of this galaxy, the characteristic ‘clumpy’ morphology observed in massive high-redshift disks arises because the disk is violently gravitationally unstable. However, many clumps are short-lived, continuously re-forming after they are disrupted by feedback.

* One word of warning: these last movies occasionally exhibit some obvious ‘ringing’ of dense regions. This is entirely an artifact of how they were made (the interpolation between snapshots), rather than a physical effect, and is being fixed.

To see more details of how each disk galaxy is constructed, and how they behave in isolation (i.e. outside of a merger), click here to check out these movies of the same systems before they encounter these mergers. This page also describes in more detail the effects of different forms of stellar feedback on the galaxies.

In the mergers here, the initial disk galaxies are on a parabolic orbit; the galaxies lose angular momentum to the dark matter halos and each other via resonant gravitational torques and sink to the center. Upon “first passage,” torques exerted by the systems drives some gas to the galaxy centers and into strong shocks in tidal and “bridge” regions between the galaxies, which drives a burst of star formation. Feedback from the massive stars formed blows a powerful super-wind in response, containing a broad mix of hot X-ray emitting gas, warm ionized gas, and dense molecular material. When the two galaxies coalesce, an even stronger burst of star formation is driven, and the ensuing super-wind is even more pronounced. However, not all the material in the winds is completely unbound, and much of this “rains down” onto the galaxy after the merger, providing the fuel to rapidly re-build rotating disks in some of the (more gas-rich) mergers. The more gas-poor systems have exhausted their gas, and the merger remnant gradually relaxes and its colors redden, as it evolves to resemble an elliptical galaxy.

To compare to merger simulations which include the effects of feedback from the accretion onto the central black hole, but do not resolve the effects of stellar feedback, click here.

The major forms/mechanisms of stellar feedback acting in these simulations are:

Radiation pressure: Light (mostly from the youngest stars) scatters off gas and dust in the galaxy. Each time a photon scatters or is absorbed, it imparts some of its momentum to that gas, “pushing” away the gas and dust. This does not “heat up” the gas, but can impart an enormous amount of momentum.

Stellar winds: Young stars blow winds off their surface that can have velocities as large as ~1000 km/s. This shocks and provides a large amount of thermal energy to heat the gas. Older stars blow “slow” winds at just ~10 km/s, but the total mass recycled into the ISM can be very large, ~30% of the original mass in stars.

Photo-Ionization: The light from the stars also ionizes gas, heating it up to ~10^4 K. These ionized ‘bubbles’ can push on the gas significantly in very low-mass galaxies (where the corresponding velocities of the gas are comparable to the disk orbital velocities). It can also destroy molecules, a critical ingredient for the next generation of star formation.

Supernovae: After a few million years, massive stars begin to explode as supernovae. Each such event imparts a large energy to the nearby ISM. Many “overlapping” events can build up huge hot bubbles of gas that generate pressures sufficient to “blow out” of the disk and vent material into the intergalactic medium.